8/9/2019 CPE 633 Lecture W2.pdf http://slidepdf.com/reader/full/cpe-633-lecture-w2pdf 1/57 SETTING PINCH DESIGN TARGETS By Siti Shawalliah Idris, AMIChemE SETTING PINCH DESIGN TARGETS
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SETTING PINCH DESIGNTARGETS
By Siti Shawalliah Idris, AMIChemE
SETTING PINCH DESIGN TARGETS
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Setting the Energy Targets
Setting the Pinch Design Targets
COMPOSITECURVE
PROBLEMTABLE
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Setting the Targets
Title of your presentation
StreamData - !Tmin
PinchAnalysis
EnergyTargets
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Try This
Setting The Energy Targets for Pinch Design
! Do you think the amount heat required is reallynecessary?
STEAM COOLING
WATER
UNITS
1200 360 4
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Enthalpy Balance
Setting The Energy Targets for Pinch Design
180o
100o
130o 40o
80o
120o 30o
60o
!H = 2000
!H = 3600
!H = -3240
!H = -3200
"#$
QH
1200
Qc 360
QH – QC =% (!H)= Q
recovery
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Setting The Energy Targets for Pinch Design
! Heat recovery is constant
! It’s necessary/possible to reduce the amount of QH
or QC
QH
%!H
QH
Qc
%!H
Qc
QH – QC =% (!H) = constant
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Enthalpy Balance
Setting The Energy Targets for Pinch Design
! Consider the lowest utilities requirement..
! Is this the best design?
No. Violation of !Tmin
QH
840
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T-H Diagram
Setting The Energy Targets for Pinch Design
! Represent a stream data
! !" is constant
T
H
Ts
Tt
130o
40o
!H = 3600
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
Mass Specific Heat Initial Final Heat
flowrate heat capacity (supply) (target) load
W (kg/s) capacity flowrate temperature temperature H (kW) CP (kJ/kgK) CP (kW/K) TS (°C) TT (°C)
Cold 0.25 4 1.0 20 200 180
stream
Hot 0.4 4.5 1.8 150 50 180
stream
Cooler
Heater200°
Product
Feed
Reactor
150°50°
20°H
C
! Consider a Two – Stream Heat Recovery Problem
Can we reduce energy consumption? Yes
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
!
Recover some heat from the hot stream" use it toheat the cold stream in a heat exchanger
!
Thus, less steam and water to satisfy the remaining
duties.
! Ideally – Recover all of 180 kW in the hot stream toheat the cold stream.
!
However, this is not possible because of temperaturelimitations.
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Setting The Energy Targets for Pinch Design
Heat
Exchanger
E
Heater
Feed
Product
Cooler
Reactor
H
C
200°
150°50°
20°
Heat Recovery Problem
! Ideally.. This is how it should look like on a PFD
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
! By the Second Law of Thermodynamics, we can’t use a hotstream at 150°C to heat a cold stream at 200°C!
As in the informal statement of the Second Law:
“you can’t boil a kettle on ice”
! So the question is, how much heat can we actually recover, howbig should the exchanger be, and what will be thetemperatures around it?
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
0
50
100
150
200
250
0 50 100 150 200 250
Heat load (kW)
T e m p e r a t u r e ( ° C )
Heating duty50 kW
Heat recovery130 kW
Cooling duty50 kW
Cold streamHot stream
!Tmin =0
! Plot T/H diagram
!
Is it possible to an approach temperature (!Tmin) = 0 for a heat exchanger design?
! !&min = 0" Infinitely large area of heat transfer
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
0
50
100
150
200
250
0 50 100 150 200 250 300
Heat load (kW)
Temperature difference∆T min 20°C
T e m p e r a t u r e ( ° C )
Cold stream
Hot stream
Heating duty70 kW
Heat recovery110 kW
Cooling duty70kW
! Streams can be shift horizontally" allow temp. difference
! !" is constant = 180kW
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
Stream Type Supply
Temp.
(oC)
Target
Temp.
(oC)
!H
(MW)
1
Cold
40
110
14
2 Hot 160 40 -12
Another example….
160oC
40oC
50oC40oC
95oC
110oC
T
H (MW)
QHmin = 3 MWQCmin = 1 MW
40o
QRec = 11MW
110o
Steam
CW
!Tmin = 10o
160o
20o
180o
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
! Shift cold stream horizontally – Increase !Tmin
T
H (MW)
QHmin = 4 MWQCmin = 2 MW
40o
QRec = 10MW
110o
Steam
CW
!Tmin = 20o
160o
20o
180o
160oC50oC 40oC
95oC
110oC
40oC
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Heat Recovery Problem
Setting The Energy Targets for Pinch Design
Two Basic Facts
! Correlation between !Tmin and QHmin and QCmin
! This means that if we choose a value of !T min, we have an energy
target for how much heating and cooling we should be using if wedesign our heat exchanger correctly.
! More in, More out
! the hot utility load is increased by any value ! , the cold utility isincreased by ! as well.
! As the stream heat loads are constant, this also means that the heat
exchanged falls by ! .
QHmin + ' " QCmin +'
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Heat Recovery Problem
Title of your presentation
! So far…
! We have seen how to apply single stream of hot
and cold.
! What if…multiple hot streams and cold streams?
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COMPOSITE CURVE
Setting The Energy Targets for Pinch Design
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Setting The Energy Target
StreamData - !Tmin
Compositecurves
EnergyTargets
Setting The Energy Targets for Pinch Design
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Composite Curve
Setting The Energy Targets for Pinch Design
! The resulting T /H plot of multiple hot and coldstreams on a single curve known as a composite
curve.
! A hot composite curve a single plot of all the hot
streams and a cold composite curve of all the cold
streams in a particular problem.
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Composite Curve
Setting The Energy Targets for Pinch Design
! Example
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Composite Curve
Setting The Pinch Design Targets
! Stream data for the flowsheet
Stream
Type
Supply
Temp. Ts
(oC)
Target
Temp. TT
(oC)
!H (kW)
Heat Capacity
Flowrate CP
(kW oC-1)
Reactor 1 Feed
Cold
20
180
3200
20
Reactor 1
Product
Hot
250
40
-3150
15
Reactor 2 Feed Cold 140 230 2700 30
Reactor 2Product
Hot
200
80
-3000
25
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Composite Curve
Setting The Pinch Design Targets
1•
Plot T/H diagram for Hot Streams and Cold Streams Separately
2•
Combine all the hot streams to get composite hot stream
3•
Combine all cold streams to get composite cold stream
4
•
Combine composite hot and cold streams together to give target for
utilities and recovery
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Composite Curve
! T/H diagram for hot streams ! Composite Hot Stream
Setting The Pinch Design Targets
T (oC)
H (MW)
80o
3150
200o
250o
40o
3000
6150
T (oC)
H (MW)
80o
Q=CP!T=4800
200o
250o
40o
6150
750
600
Add heat available in each temperature interval to formcomposite curve
H intervals
(250-200)*15
(200-80)*(15+25)
(80-40)*(15)
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Composite Curve
! T/H diagram for cold streams ! Composite Cold Stream
Setting The Pinch Design Targets
T (oC)
H (MW)
140o
3200
180o
230o
20o
2700
5900 5900
2000
Similar approach for cold streams to obtain cold compositecurve
T (oC)
H (MW)
140o
2400
180o
230o
20o
1500
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Composite Curve
Setting The Pinch Design Targets
! Plot of hot and cold composite curves together gives the targetfor hot at cold utility
Shift the two curves so that will get !Tmin = 10oCT (oC)
H (MW)
50
QREC=5150
250
0
100
150
200
Pinch
!Tmin =10
Qhmin = 750
Qcmin = 1000
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Composite Curve
Setting The Energy Targets for Pinch Design
! Shifting of the curves leads to behavior similar tothat shown by the two-stream problem.
! Though, the “kinked” nature of the composites
means that !T min can occur anywhere in the inter-change region and not just at one end.
For a given value of !T min, the utility quantitiespredicted are the minima required to solve the heat
recovery problem.
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Composite Curve
Setting The Pinch Design Targets
! Although there are many streams in the problem..
In general !T min occurs at only one point of closest approach, which iscalled the pinch (Linnhoff et al. 1979).
! This means that it is possible to design a network which uses theminimum utility requirements, where only the heat exchangers at thepinch need to operate at !T values down to !T min.
!
It will be seen later that the pinch temperature is of great practicalimportance, not just in network design but in all energy-relatedaspects of process optimisation.
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Composite Curve
Setting The Energy Targets for Pinch Design
! Let’s try increase !Tmin =20oCT (oC)
H50
QREC=4750
250
0
100
150
200
Pinch
!Tmin =20
Qhmin = 1150
Qcmin = 1400
T (oC)
H (MW)
50
QREC=5150
250
0
100
150
200
Pinch
!Tmin =10
Qhmin = 750
Qcmin = 1000
QC and QH increasecorrespondingly
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Economic Trade-off
Setting The Energy Targets for Pinch Design
! The correct setting of !Tmin fixed by economictrade-offs
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Summary
Setting The Pinch Design Targets
! Temperature-enthalpy diagrams can be used todetermine heat recovery potential
! Composite curves can be used to target for many hotstreams and many cold streams
! Energy targets set from material and energy balanceandΔTmin
!
Can be varied to different targets
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Setting Energy Target – Composite Curve
WORKING EXAMPLE
Setting The Energy Targets for Pinch Design
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Example 1
Title of your presentation
! Consider a process
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Example 1
Streams No &
Type
Suppy
Temperature , Ts
(oC)
Target
Temperature, Tt
(oC)
Heat Capacity
Flowrate (kW/oC)
1 Hot 180 80 20
2 Hot 130 40 40
3 Cold 60 100 80
4 Cold 30 120 36
!Tmin =10 oC
Construct the composite curves
Read off energy targets for !Tmin =10 oC
35
Setting The Pinch Design Targets
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PROBLEM TABLE ALGORITHM
© your company name. All rights
reserved.Setting The Pinch Design Targets
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Recap
StreamData - !Tmin
Compositecurves
•
Complicated nature
EnergyTargets
“graph paper and scissors”approach (for sliding thegraphs relative to one
another) which would be
messy and imprecise.
Setting The Pinch Design Targets
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Alternative Solution
Setting The Pinch Design Targets
The Problem Table Method
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Setting The Pinch Design Targets
Divide into temperature intervals:Within a particular interval the two composite curves are not !Tmin apart from
each other
T (oC)
H (MW)
50
250
0
100
150
200
Pinch !tmin $( !Tmin
Within interval
Temperature Intervals
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Title of your presentation
To allow for the maximum possible amount of heat exchange within each
temperature interval.
The only modification needed is to ensure that within any interval, hot streams
and cold streams are at least !T min apart.T (oC)
H (MW)
50
250
0
100
150
200
Shifted curves
!Tmin
Setting up the intervals in this way guarantees that full heat inter-change within anyinterval is possible.
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Setting The Pinch Design Targets
! This is done by using shifted temperatures.
Shifting Rule:
Cold stream : + "!T min
Hot stream : -"!
T min
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Setting The Energy Targets for Pinch Design
! For example:
!T min = 10oC
Thus, "!T min = 5oC
Actual temperatures Shifted temperatures
Stream number and type CP (kW/K) TS
(°C) TT
(°C) SS
(°C) ST
(°C)
1. Cold 2 20° 135° 25° 140°
2. Hot 3 170° 60° 165° 55°
3. Cold 4 80° 140° 85° 145°
4. Hot 1.5 150° 30° 145° 25°
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2
4
1
3
165°C
145°C
140°C
85°C
55°C
25°C
170°C
150°C
145°C135°C135°C
90°C
60°C60
°C50
°C
80°C80°C 90°C
145°C
150°C140°C
30°C20°C
1
2
3
4
5
Setting The Energy Targets for Pinch Design
! Shifted temperatureintervals
! Arranged Ts* and Tt*
from highest to lowest
! Boundary is where the
stream starts or end! Min 2 boundaries
Real temperature
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Setting The Energy Targets for Pinch Design
In each shifted temperature interval, calculate energybalance from
!Hi = (% CPH-% CPC )i !Ti
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Setting The Energy Targets for Pinch Design
! Stream Data
Surplus
Interval Si Si1 ∑ CPHOT ∑ CPCOLD or
number i (°C) (kW/°C) ∆Hi (kW) deficit
S 1 165°C1 20 3.0 60 Surplus
S 2 145°C
2 5 0.5 2.5 Surplus
S 3 140°C
3 55 1.5 82.5 Deficit
S 4
85°C 4 30 2.5 75 Surplus
S 5 55°C
5 30 0.5 15 Deficit
S 6 25°C
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Setting The Energy Targets for Pinch Design
! It would therefore be possible to produce a feasiblenetwork design based on the assumption:
All “surplus” intervals rejected heat to cold utility,
All “deficit” intervals took heat from hot utility.
However, this would not be very sensible, because it wouldinvolve rejecting and accepting heat at inappropriatetemperatures.
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T 145°C ∆T min /2For all hot streams
T 145°C ∆T min /2For all cold streams
60kW
60kW145°C
2
1
!
Instead of cool down surplus energy
by using cooling water (cold utility),
the surplus heat is cascaded to the
next temp. interval.
!
Assuming no hot utility is supplied
(!H =0) to the hottest interval 1;
can set up cascade.
Setting The Energy Targets for Pinch Design
Key feature of the temperatureintervals:
‘Any heat available in interval i is hotenough to supply any duty in
interval i+1
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From hot utility
0 kW
60kW
62.5 kW
20kW
55kW
40kW
To cold utility
1
2
3
4
5
165°C
145°C
140°C
85°C
55°C
25°C
∆H
60 kW
∆H
2.5 kW
∆H
82.5 kW
∆H 75 kW
∆H 15 kW
(a) Infeasible
Setting The Energy Targets for Pinch Design
Cascade any surplus heat from high temperature to low temperatureFrom hot utility
0 kW
60kW
62.5 kW
20kW
55kW
40kW
To cold utility
1
2
3
4
5
165°C
145°C
140°C
85°C
55°C
25°C
∆H 60 kW
∆H
2.5 kW
∆H 82.5 kW
∆H 75 kW
∆H 15 kW
(a) Infeasible
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Heat flows cannot be negative! Add heat to make them at least 0!
Title of your presentation
From hot utility
82.5 kW
0 kW
75kW
60kW
20kW
80kW
To cold utility
1
2
3
4
5
∆H
60kW
∆H 2.5 kW
∆H 82.5 kW
∆H 75kW
∆H 15kW
(b) Feasible
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Setting The Energy Targets for Pinch Design
! Minimum Hot Utility Requirement = 20 kW
! Minimum Cold Utility Requirement = 60 kW
! Pinch temperature
Hot Stream Pinch Temp = 85o
C + 5o
C = 90o
CCold Stream Pinch Temp = 85oC – 5oC = 80oC
! Maximum Heat Recovery = 450kW! Hot streams loads = -510 kW( -ve indicates heat rejects)
! Max Heat Recovery = 510-60 = 450 KW!
Cold streams loads = 470kW
! Max. Heat Recovery = 470 – 20 =450 kW
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Summary
Setting The Energy Targets for Pinch Design
! The problem table algorithm:
Step 1: Adjust for !Tmin
Step 2: Set up temperature intervals
Step 3: Calculate interval heat balance
Step 4: Cascade for positive heat flows
Then, QHmin, QCmin and pinch locationwithout drawing graphs
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Setting The Energy Targets – Problem Table
Algorithm
WORKING EXAMPLE
Setting The Energy Targets for Pinch Design
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Setting The Energy Targets for Pinch Design
! For example:
!T min = 10oC
Thus, "!T min = 5oC
Stream Type Supply Temp.Ts (
oC) Target
Temp. TT (oC)
Shifted SupplyTemp. Ts*
Shifter TargetTemp.
Tt*
1
Cold
20
180 T*s = 20+ 5 =
25185
2 Hot 250 40 = 250 – 5 = 245 35
3 Cold 140 230 145 235
4 Hot
200
80
195 75
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© your company name. All rights
reserved.Setting The Energy Targets for Pinch Design
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Temperature Interval Heat Balance
Title of your presentation
! !
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Setting The Energy Targets for Pinch Design
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Thank You …